True Blue

Spring is in many respects a celebration of color, as plants and animals emerge from a very blah-colored winter dormancy.  We have been barraged by a profusion of pinks, reds, oranges, yellows, golds, browns, chestnuts in the past couple of weeks as the spring migrants have passed through here.  These bright colors are all products of pigments synthesized or ingested and metabolized by their exhibitors.  You’ll notice I left some colors out.

The blues:  Blue color in animals is produced by structural reflections of incoming light, not a pigment.  As a result, the true coloration on an animal often changes as their position changes relative to the observer.

The greens:  Green color is a mixture of blue and yellow, so green is a mixture of pigment and structural reflection.

Tree Swallow

Here’s how it works.  The underlying color of a blue bird is black (or brown) and is produced by melanin pigment.  Feathers that appear blue have air spaces in their pigment-containing cells that reflect short wavelengths (blue light) but transmit longer wavelengths.  Thus red and yellow (longer) light wavelengths are absorbed by the melanin pigment and blue light is amplified and reflected back to the observer’s eye.  The structural organization of the air spaces determines the intensity and hue of the “blue-ness”.

This Tree Swallow looks bluer than the previous one.  It might be older (first year birds are often more brown), or the incoming light might be oriented differently in the two birds.

This Tree Swallow looks bluer than the previous one. The bird might be older (first year birds are often more brown), or the incoming light might be oriented differently in the two birds.

Blue color ranges from purplish on the head to turquoise on the belly feathers in this Indigo Bunting.  The primary wing feathers lack those light reflecting air spaces, probably because it might weaken them for the flight function.

Blue color ranges from cobalt on the head to turquoise on the belly feathers in this Indigo Bunting. The primary wing feathers lack those light reflecting air spaces and show the underlying brown/black melanin pigment.  Modification of internal structure might weaken the feather, making it unsuitable for flight.

A similar process of light scattering and absorption explains blue colors in insects, like dragonflies.  In some species, the outer cuticle is transparent, and the scattering of blue light and absorption of longer wavelengths occurs in the layer just beneath the cuticle.  This is the basis for the blue color of the male Eastern Pondhawk.

eastern-pondhawk-blue male

In other species, the light scattering occurs in a waxy layer above the cuticle, and can be removed by dissolving the wax.  The animal then becomes the color of the underlying pigment.

There is no doubt that blue coloration is striking and noticeable in animals, and it seems to be used to call attention to some aspects of the animal’s anatomy.  What could be more striking than seeing an animal advertise this?

Male vervet monkey showing off his blue scrotum.  In Simonis and Berthier, Intechopen.com article:  "How Nature produces blue color"

Male vervet monkey showing off his blue scrotum. In Simonis and Berthier, Intechopen.com article: “How Nature produces blue color”

7 thoughts on “True Blue

  1. For someone who is colorblind like myself this is very interesting stuff. I can see all the blues here but some, like the dragonfly, look more blue green or maybe turquoise. My color finding software sees steel blue and slate gray.

    • I agree — the dragonfly is kind of slate blue bordering on turquoise. Color blindness aside, I know that even my husband and I don’t agree on what to call various shades of blue and gray. I think everyone sees colors slightly differently, depending on the composition and distribution of cones in their retina.

  2. Pingback: Mr. 23 Thorns’ discourse on “Blue” | Back Yard Biology

  3. Sue, just found the last blue bird that you posted ( the one with blue, turquoise, and brown).Wondering if you know what to call them?

    • The last bird is an indigo bunting male. They are a late spring migrant here in MN, and probably won’t be arriving for another two months.

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